An intriguing property of a three-dimensional topological insulator (TI) is the existence of surface states with spin-momentum locking. We report the discovery of a new type of Hall effect in a TI Bi2Se3 film [1]. The Hall resistance scales linearly with both the applied electric and magnetic fields and exhibits a π/2 angle offset with respect to its longitudinal counterpart, in contrast to the usual angle offset of π/4 between the linear planar Hall and anisotropic magnetoresistance. At variance with the nonlinear Hall effect due to Berry curvature dipole in time-reversal invariant materials, this novel nonlinear planar Hall effect originates from the conversion of a nonlinear transverse spin current to a charge current due to the concerted actions of spin-momentum locking and time-reversal symmetry-breaking, which also exists in other non-centrosymmetric materials [e.g., WTe2 and the 2DEG on the SrTiO3(001) surface] with a large span of magnitude.
Spin-orbitronics, which takes advantage of spin-orbit coupling (SOC), has expanded the research objects of spintronics to nonmagnetic materials. Here, we report the emerging nonlinear spintronic phenomena in the inversion-asymmetric nonmagnetic materials with SOC. For instance, the surface state of three-dimensional topological insulator (TI) owns helical spin textures with the spin and momentum perpendicularly locked. We show the observation of a nonlinear magnetoresistance (called bilinear magneto-electric resistance, BMER) and nonlinear Hall effect in a prototypical TI Bi2Se3, which scale linearly with both the applied electric and magnetic fields. We further reveal that these effects are originated from the conversion of a nonlinear spin current to charge current under the application of an external magnetic field. A close link between the BMER and the spin texture was established in TI surface states, which enables a novel transport probe of spin textures. We further extended the observation of BMER effect to the d-orbital two-dimensional electron gas (2DEG) at a SrTiO3 (STO) (111) surface. The BMER probes a three-fold out-of-plane spin texture, in addition to an in-plane one at the STO(111) surface 2DEG. This novel spin texture is in contrast to the conventional one induced by the Rashba effect. By performing tight-binding supercell calculations, we find that this 3D spin texture is fully described by the confinement effects of the STO t2g conduction band in the (111) plane. These findings open a new branch in spintronics, which discusses the nonlinear transport effects in spin-polarized nonmagnetic materials, and is therefore referred to as nonlinear spintronics.
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